This invention relates to an efficient encoder for efficiently encoding a moving image signal with a less quantity of codes and an efficient decoder therefor in various equipments such as recording/transmission equipments, or other display devices, etc., used for signal processing of digital signals, and more particularly to an interfield predictive encoder for a moving picture signal subjected to interlace scanning and a decoder therefor.
As the coding system of utilizing correlation between frames of a moving signal of efficient coding systems of encoding a moving picture signal with a less quantity of codes, there is known an interframe predictive coding.
This interframe predictive coding system performs a predictive coding by drawing attention to the fact that ordinary moving pictures are fairly similar to each other between frames. Namely, a procedure is employed to predict a signal of a frame subject to coding from a signal of a last coded frame to encode only a predictive error (residual).
In the case of the interframe predictive coding, however, if an image varies to much degree by movement of an object or scene change, suitable prediction may fail to be conducted. Accordingly, there are instances where it is desirable to carry out encoding within a frame (field) without conducting interframe prediction.
In view of this, there have been conducted studies on the adaptive predictive coding to carry out interframe prediction in the case where interframe prediction is considered to be suitable, and to carry out interframe (intrafield) prediction in the case where otherwise. In recent years, a motion compensated interframe predictive coding to carry out prediction by altering predictive methods in correspondence with the motion of an image becomes popular.
As an example, there is announced in "15/30 Mb/s motion compensated interframe/interfield/interframe adaptive predictive coding system" Journal of the Institute of Television Engineers, Vol. 39, No. 10, pp. 861 (11) to 868 (18) (October, 1985), etc.
This system encodes a television relay signal at a relatively high rate. For a moving picture signal subjected to interlace scanning, the interfield (intraframe) prediction is used in addition to the motion compensated interframe prediction and the intrafield prediction.
In the case of attempting to decode a certain frame, such a system being studied in the communication system requires all past data because data is a stack of past data. Accordingly, in the case where there is a necessity of carrying out decoding from an arbitrary place by random access or high speed search, etc. in the storage system media, the interframe prediction must be reset on the basis of a fine unit (4 to 8 frames).
However, if the interframe prediction is reset on the basis of a fine unit, an original image signal is subjected to intraframe coding as it is, leading to lowered coding efficiency. On the other hand, in the case of a reverse playback for carrying out playback in an order, in terms of time, opposite to that in the case of an ordinary playback, predictive values for decoding cannot be provided by the conventional prediction by the last or preceding frame, disadvantageously resulting in failure to decoding.
In view of this, as the means for solving the problems with such a conventional recursive interframe predictive coding, the inventor of the present invention has already inverted a method positively utilizing, for the purpose of random access or high speed search, the fact that independent frames cyclically exist to make a prediction for frames therebetween by using preceding and succeeding (old and new) independent frames on the basis of the linear prediction, etc., thus to improve the coding efficiency. The patent application therefor was filed by this applicant (Japanese Patent Application No. 11587/89 -Japanese Patent Application Laid Open No. 192378/90-), entitled "Interframe predictive encoding system", which corresponds to U.S. patent application Ser. No. 07/465,747 patented on Jan. 15, 1991 "Interframe predictive encoding system with encoded and transmitted prediction error").
"Interframe predictive encoding system" according to the earlier patent application by this applicant (hereinafter referred to as "earlier application") sets in advance independent frames at a fixed interval to make a prediction for frames therebetween by preceding and succeeding (old and new) independent frames, thus to encode them. Accordingly, random access can be provided using the interval between independent frames as a unit in the storage system media. Thus, high speed search can be conducted without wastefulness and prediction in conformity with changes in an image can be conducted. In addition, since S/N ratio of the predictive signal is also improved, a higher precision predictive signal can be provided. Further, since coded data thus obtained are of symmetric structure on the time base, reverse playback can be advantageously realized with ease. In addition, since this predictive encoding system is not cyclic prediction as in the prior art, the decoding processing on the encoder side can be eliminated.
Meanwhile, the above-described conventional predictive encoding system has the following problem in encoding an interlaced signal in addition to the above-described problems.
Namely, the drawbacks with respective predictions in "15/30 Mb/s motion compensated interframe/interfield/intraframe adaptive predictive encoding system" disclosed as the prior art are as follows. First, the interfield prediction is only effective in the case where motion of an image is to an extent corresponding to 1/2 line, but is not so advantageous in the case of a still picture or in the case where there is any motion more than 1/2 line. Furthermore, the motion compensated interframe prediction is carried out every field, and each field includes aliasing due to line thinning of interlaced signals. As a result, the interfield prediction cannot necessarily make a prediction.
On the other hand, when coding of the earlier application (Japanese Patent Application Laid Open No. 192378/90) by this applicant is applied to interlaced signals as it is, the following problems would arise.
FIG. 1 is a view showing the field configuration in the case where the interval (N) between independent frames in the earlier application is 5 frames.
In this figure, black circles and white circles represent lines of fields independently encoded and lines of fields subjected to interfield prediction and encoding, respectively. Further "o" and "e" represent odd fields and even fields, respectively.
In independently conducting encoding within a frame, since there is any time change between fields, if a pattern of an image is moving, a double image results. Thus, coding efficiency is lowered to much extent.
Further, in the case of reproducing or playing back only independent frames to make a high speed search, respective fields are reproduced in order of a, b, c and d in FIG. 1. Accordingly, motion of an image between respective fields and motion of an image between independent frames are equidistantly reproduced. As a result, motion of the image become unnatural. If an approach is employed to reproduce fields on only one side (i.e., only odd fields, or only even fields), motion of the image becomes natural, but the vertical resolution is reduced to one half.